The present invention relates to a plasma-generating spark plug used in particular for the ignition of internal combustion engines using electric sparks between the electrodes of a spark plug.
More specifically, the invention relates to an internal combustion engine radio-frequency spark plug comprising two plasma-generating electrodes separated by an insulator, it being possible for one of the two electrodes to consist of the entirety of the cylinder head and of the shell of the spark plug.
Plasma-generating spark plugs are high-frequency multi-spark ignition systems capable of providing ignition in spark-ignition engines under the best conditions while at the same time reducing polluting emissions, particularly under lean burn conditions. They are liable to coking, particularly when cold.
Like all spark plugs, they are characterized by a thermal index. This index takes account of their thermal behavior at particular engine operating points. In particular, it provides an indication as to their ability to withstand temperatures that are high enough to avoid coking by pyrolysis, without suffering from “pre-ignition”.
Publications FR 2859830, FR 2859869 and FR 2859831 disclose a multi-spark spark plug known as a cold spark plug because it does not come up to temperature quickly enough to avoid coking. Indeed, the buildup of a deposit of carbon or coke on the electrodes is observed with such spark plugs and this significantly reduces the insulation needed between the tip of the central electrode and the shell. With poor insulation, there is a risk that the high-voltage power applied to the spark plug might not be high enough to be able to cause the necessary “breakdowns” that trigger the sparks.
To avoid coking, particularly when cold, of the spark plug electrode exposed to the atmosphere of the combustion chamber, one solution might be to increase the temperature of the insulator, to encourage the destruction of deposits through a pyrolysis phenomenon. This temperature is dependent on the thermal resistance of the spark plug as a whole, including its insulator.
The steps usually taken to increase the temperature of the insulator are limited by the onset of “pre-ignition” at the spark plugs, when these reach excessively high temperatures during operation.